How to bias the Antique Sound Lab's 1009 monoblock

Hello out there. Can anyone help me? The last week, I bought a pair of Antique Sound Lab's 1009 power amps. These are tube monoblocks. But there's no manual!! Can anyone help me to bias this thing? It has three control knobs, plus a read-out meter. The knobs are labeled Vb1, Vb2, and Vb1-Vb2. The last knob is just a switch between Vb1 and Vb2.

Thanks in advance.

Bill